1. Field of the Invention
The present invention relates to a polymer electrolyte composition, a method for preparing the same and a lithium secondary battery employing the same, and more particularly, to a polymer electrolyte composition, a method for preparing the same and a lithium secondary battery employing the same which has a high ionic conductivity, good mechanical properties, a stable interface characteristic, an efficient and good discharging characteristic at high and low temperatures.
2. Description of the Prior Art
There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as cellular communication, satellites, portable computers and electric vehicles. Particularly, much effort has been dedicated to the development of a lithium ion battery having an cathode including lithium, a anode including lithium or carbon and a non-aqueous electrolyte, because of its higher energy density than that of a lead storage battery or nickel-cadmium battery having an aqueous electrolyte.
Recently, the widely used lithium ion battery which has a satisfactory ionic conductivity utilizes a liquid electrolyte, however, there is frequent occurrence of leakage of the liquid electrolyte out of the case. Moreover, any leakage in the cell lessens the performance of the battery. Accordingly, lithium ion batteries are packaged by utilizing an aluminum can and are provided with various protective devices, thereby enlarging the volume of the cell, resulting in an undesirable reduction of energy density. Further, such lithium ion battery is not applicable to a battery having a thickness of 3 mm or less.
In contrast, solid electrolytes are free from problems of leakage, however, they tend to exhibit inferior properties compared to the liquid electrolytes. This is attributed by the fact that ionic conductivities for the solid electrolytes are often 5-100 times poorer than for the liquid electrolytes.
Generally, a polymer lithium secondary battery includes an anode, a polymer electrolyte and a cathode. The components are selected for satisfying various conditions for the secondary battery such as lifetime, capacity, temperature characteristic, stability, etc.
As for the components for the cathode applied to the secondary battery, lithium oxide complex (LiCoO2, LiMn2O4, LiNiO2) which has a laminated structure and lithium ion can be inserted/separated between layers, can be utilized. As for the components for the anode, carbon compounds such as graphite compounds or coke can be utilized. Examples of which include mesocarbon microbean (MCMB) and mesophase carbon fiber.
Such polymer electrolyte widely utilized as main component of the lithium battery which is free from the problem of leakage. The manufacturing of the battery utilizing the polymer electrolytes is advantageous. However, the polymer electrolyte is required to have a good ionic conductivity, a thermal and electrochemical stability, a good mechanical strength and a good adhesiveness to the electrodes.
The polymer electrolyte currently utilized or under development include a main liquid-type organic solvent such as ethylene carbonate and propylene carbonate, a vice liquid-type organic solvent such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, and polyvinylidene difluoride-based compounds (PVdF), polyacrylonitrile (PAN), polyethylene oxides, a copolymer thereof or a mixture thereof, which can accept lithium salts such as LiPF6 and LiAsF6.
The polymer electrolyte including the polyvinylidene fluoride compounds has a good mechanical strength. However, the adhesiveness to the electrodes is not sufficient. This necessitates an adhering process by utilizing heat or pressure. During the adhering process of the electrodes with the electrolyte, the solvent may evaporate. Thus, films not containing the electrolyte are adhered to the electrodes and then additional impregnation process into the solvent is implemented.
When polyacrylate polymer electrolytes having a good affinity to the solvent are utilized in order to increase the adhesiveness to the electrodes, a good adhesiveness to the electrodes can be accomplished; however, the electrolytes exhibit a poor mechanical strength.
One class of polymer electrolytes, specifically gel electrolytes in which liquid electrolyte is dispersed in a polymer matrix, includes a significant fraction of solvents in addition to the salt is contained in the polymer matrix.
A method for preparing the gel electrolytes is disclosed in U.S. Pat. No. 5,456,000. A battery is assembled with a gel electrolyte to leave the electrolyte salt and solvent out of the polymer gel system until after the cell is completely fabricated. Thereafter, the solvent and the electrolyte salt may be introduced into the polymer system in order to swell the battery. This battery has an advantage of allowing the cell to be fabricated in a non-dry environment. However, a film is formed from a polymer containing a plasticizer in order to facilitate the impregnation of the polymer film with the solvent and then the battery is assembled. The plasticizer is then extracted out to form a microporous film and the solvent used for the extraction is evaporated. Such process requiring homogeneous impregnation of the polymer with the solvent requires many hours which lengthens the processing time.
In order to overcome the above-described problem, U.S. Pat. No. 5,219,679 discloses a method for preparing a polymer electrolyte after mixing polymer and solvent. By this method, the solvent is already homogeneously dispersed into the polymer prior to the assembling of a battery. As such, an additional process of extraction of a plasticizer or drying is not necessary. However, the preparation of the polymer electrolyte and the assembling of the cell should be implemented under a dry condition. Further, when the polymer electrolyte contains a large amount of solvent, its mechanical strength is poor, which makes a continuous processing is difficult and an electrical short is liable to generate.
U.S. Pat. Nos. 5,585,039, 5,639,573, 5,716,421 and 5,688,293 disclose polymer electrolytes prepared by filling polymer electrolytes into porous films having good mechanical strength to overcome the problems of the mechanical strength. According to these methods, a filling process or a coating process of the electrolyte into or onto the porous film is additionally implemented, thus complicating the manufacturing process of the battery.
Therefore, it is preferred that a gel polymer electrolyte containing a polymer and a solvent is prepared, and then an anode, a cathode and thus obtained polymer electrolyte are integrated to manufacture the battery, which simplifies the manufacturing process of the battery.
For this case, since only one coating process is necessary for the preparation of the polymer electrolyte, ionic conductivity, mechanical strength of a mixture state of the polymer and the solvent and the interface adhesiveness to the electrodes are anticipated to exhibit good qualities. Since the polymer electrolyte impregnated with the solvent is integrated, a lamination method at a high temperature cannot be utilized. Accordingly, the polymer electrolyte should have a good interface adhesiveness to the electrodes.
U.S. Pat. No. 5,849,433 discloses a method for preparing a polymer electrolyte by utilizing a material having a good mechanical strength and adhesiveness in order to improve the mechanical properties. According to the method, the polymer electrolyte is prepared by forming a film from a mixture of materials having good mechanical strength and adhesiveness to obtain a desired polymer electrolyte and then impregnating the film with a liquid electrolyte.
However, in this method, additional impregnation process of the polymer film with the liquid electrolyte is necessary to complicate the manufacturing of the polymer electrolyte.
Accordingly, it is an object of the present invention to provide a polymer electrolyte composition having a good mechanical strength and adhesiveness.
Another object of the present invention is to provide a method for preparing a polymer electrolyte having a good mechanical strength and adhesiveness.
Further another object of the present invention is to provide a lithium secondary battery employing a polymer electrolyte having a good mechanical strength and adhesiveness.
To accomplish the object, there is provided in the present invention a polymer electrolyte composition comprising a polymer mixture and a solvent in which a lithium salt is dissolved. The polymer mixture includes polyvinylidene fluoride-based polymer and at least one polymer selected from the group consisting of polyacrylonitrile and polymethyl methacrylate.
Another object of the present invention can be accomplished by a method for preparing a polymer electrolyte composition in which a polymer mixture including polyvinylidene fluoride-based polymer and at least one polymer selected from the group consisting of polyacrylonitrile and polymethyl methacrylate, and a solvent in which a lithium salt is dissolved, are mixed in a mixing ratio range of 1:3-10 by weight, then, heating the mixture.
Further object of the present invention can be accomplished by a lithium secondary battery including a polymer electrolyte composition comprising a polymer mixture and a solvent in which a lithium salt is dissolved. The polymer mixture includes polyvinylidene fluoride-based polymer and at least one polymer selected from the group consisting of polyacrylonitrile and polymethyl methacrylate.
Polyvinylidene fluoride-based polymer includes a large amount of solvent and lithium salts and provides a good mechanical strength. Polymethyl methacrylate polymer has a good affinity to the solvent which strongly adheres electrolyte to the electrodes. Polyacrylonitrile polymer has a good adhesiveness to the electrolyte, thus it improves the adhesiveness of the electrolyte to the electrodes without deteriorating the excellent mechanical properties of the polyvinylidene fluoride-based polymer.
According to the present invention, an electrolyte having good mechanical properties and a good adhesiveness to the electrodes is mixed with a solvent to obtain a polymer electrolyte in which the phases of the polymer and the solvent are separated. Therefore, the mechanical strength of the electrolyte can be increased and the affinity of the electrolyte with the solvent can be improved to minimize the leakage and evaporation of the solvent in the polymer electrolyte. As a result, a lithium secondary battery having a stable charge/discharge characteristic and a high capacity can be manufactured. Further, since the electrolyte of the present invention has a good adhesiveness to the electrodes, a contacting resistance of thus obtained battery is small and a local concentration of a current can be prevented to improve the performance of the battery during charge/discharge thereof.